Single photon counting is useful for many applications, including the measurement of quantum states in tomography or in quantum key distribution systems, as well as in a wide variety of other applications including light-based ranging. Avalanche photodiodes (APDs) are attractive since they are inexpensive, small, and convenient to use. In order to detect single photons thereby acting as a single photon detector (SPD), the bias voltage of an APD is typically brought above the breakdown level, at which point a single photon can set off a macroscopically detectable breakdown event. This is the Geiger mode of operation. Often the bias voltage is time-gated above the breakdown only when optical pulses arrive in order to get acceptable performance (D. S. Bethune et al., “System for gated detection of optical pulses containing a small number of photons using an avalanche photodiode” U.S. Pat. No. 6,218,657).
Applying a time-gated voltage across the APD causes a charge to feed-through the device that makes detecting small breakdowns difficult. However, the capability to limit the size of the breakdowns is beneficial since large breakdowns correspond to large charge flows through the device, causing more trapped carriers which in turn cause an unwanted afterpulse effect where the device can break-down upon receiving a gate pulse even when no photons are present. This afterpulse effect can be controlled by waiting a suitably long time between gates to allow the carriers to disperse. However that slows down operation.
Recent work in the field has suggested that the use of either a sine wave gate or the use of differential subtraction can allow small breakdowns to be detected using suitable analog processing (N. Namekata et al., “800 MHz single-photon detection at 1550-nm using an InGaAs/InP avalanche photodiode operated with a sine wave gating,” Optics Express 14, No. 21, Oct. 16, 2006 and US patent application No. 20100111305 A1 by Yuan et al.). This has allowed for much faster gating rates. These methods are fairly simple in that they effectively use analog processing techniques. This reduces their flexibility. It can be difficult to change the rate of operation when using the type of analog subtraction methods in the prior art. For instance, with sine wave gating analog filters are used to remove unwanted spectral components, and such filters have very specific operating frequencies. On the other hand, self-differencing subtraction methods use a true time delay such as a delay line. If the time delay is implemented as a fixed delay line the frequency of operation is limited to a discrete set of values, such as an integer number of pulse repetition times.
Digital sampling using an analog-to-digital converter (ADC) combined with a simple type of digital signal processing (averaging the sampled output over many gate cycles to determine the best threshold) was proposed to digitally process the APD breakdowns (US patent application No. 20090236501 by Takahashi et al.). In principle this is a more flexible method than purely analog methods, however the gate generation circuit is not specified as digital and no substantial control over the gate generation is exploited. Additionally, all the processing is performed in the digital domain after sampling the signal from the APD using an analog-to-digital converter (ADC). The breakdown from the APD is largely repetitive capacitive feed-through with a small breakdown signal. This poses a dynamic range issue for the sampler, since the feed-through signal will saturate the sampler before the breakdown signal does. Ideally, the input to the sampler should be primarily the desired breakdown signal and that breakdown signal should be of a large enough magnitude so that the sampler records it with a high signal-to-noise ratio. This typically means that the (possibly amplified) breakdown signal should consume a significant fraction of the dynamic range of the sampler. The aforementioned analog signal processing methods address this problem by greatly reducing the feed-through via analog processing. However, as previously mentioned analog processing usually carries with it certain limitations and reduced flexibility as well. Analog and digital processing methods have not been optimally combined in prior art. Moreover, prior art does not address estimating various metrics such as the dark count rate (dark count rate is the probability of detecting a photon when none is incident) or the detection efficiency and methods to optimize the operating parameters to optimize these metrics. In a real system these metrics are important and to some degree require a trade-off where improving one will likely degrade another.
It is also noted that often times multiple SPDs are used in one system, where the output of the multiple SPDs can each be analyzed individually or together such as when measuring coincidence counting statistics, for example when performing a quantum state tomography measurement. In such cases it is desirable to design the system as a whole so as to maximize shared resources and minimize the number of expensive components or limit the number of traces interconnecting the various electronics thereby saving printed circuit board space. APD's can also be operated in linear mode, where their output voltage is linearly related to the optical intensity. In this case the bias voltage to the APD is below the breakdown level. Control over an APD bias such as to optimize its gain in linear mode has been described by Anderson in U.S. Pat. No. 5,929,982. In that work an ADC is used to look at the detector noise with no light incident on the APD and adjust the bias to a desired noise level. This technique is used for gain control when the APD is operated in linear mode so as to address dynamic range issues and issues associated with parameter variations and temperature fluctuations of the uncooled device, as opposed to Geiger mode for detecting single photons where the gain is undefined and the devices are almost always temperature controlled. Additionally, controlling the APD in linear mode is much simpler because effects such as afterpulsing are not present and the APD bias is simply a DC bias level, and therefore issues associated with the shape, magnitude, and phase of the gate pulse, as well gate feedthrough, are absent.
What is needed is a system of digital control of the SPD such that analog processing can also be implemented and optimized appropriately, preferably over a wide range of operating conditions. Performance metrics should be automatically calculated and optimal performance automatically determined by the system with little or no manipulation by the users. It is desirable to be able to monitor performance including estimating parameters such as dark count rate, detection efficiency, and afterpulse count rate. The output of the SPD can be processed in the analog domain to remove undesired feedthrough, then sampled with a sampler. Ideally the system will require a small number (including just one) of samples per gate pulse. The system should be reprogrammable so that operation over a broad range of conditions, including a wide range of gating frequencies, is possible.
Designs which allow multiple such SPDs to be measured and controlled efficiently with the minimum number of parts and high speed inter-connections are also desired. In particular such multi-SPD systems have applications in quantum state tomography, where the ideal configuration of the SPD may change depending on the properties of the quantum state to be measured.